It is believed that certain dual-sensing intrusion detection systems, which simultaneously employ two different detection techniques may monitor a volume of space using a passive infrared sensor (PR) and a microwave detector adapted to determine a Doppler frequency shift in received radiation. The redundancy provided by the two distinct detection devices is intended to eliminate the occurrence of false alarms for certain “non-intrusion” events. For example, a spinning fan may give rise to a strong Doppler signal but may not generate significant amounts of IR radiation. In such systems, it may be advantageous to generate an alarm signal when both detection devices detect an intrusion during the same period. According to this technique, an alarm is generated by combining the output of the detection devices by an “AND” logic gate. Since the output signals from each of the detection devices is processed separately from the other and they are only combined at a final stage to reach a determination as to the presence of an intruder, this technique may be referred to as “decision-level fusion”.
The respective detection devices may have varying sensitivities with respect to different intrusion events. For example, Doppler microwave sensors may be more sensitive to radial movement while IR sensors may be more sensitive to transverse movements across a scanned area. Continual adjustment of the relevant thresholds for detection may be required for optimal performance. It is believed that a difficulty associated with this variation is in finding an optimal balance between providing sufficient sensitivity to enable detection of intrusions in most situations and avoiding false alarms.
One situation that may particularly test this balance is the movement of pets in the vicinity of the detection system. The movement of a large dog, for example, can induce and/or generate high-amplitude signals at both IR and microwave detection devices. It is believed that U.S. Pat. No. 5,670,934 is directed to this problem by allocating upper and lower IR focus zones (taking advantage of the fact that pets are normally shorter than people) and by temperature compensation to correct for the influence of ambient conditions. However, even with temperature compensation, it may be difficult to set the intrusion detection threshold for the IR signal because the IR focus zones are not necessarily perfectly positioned and do not completely differentiate between larger and smaller moving objects. In particular, when a person moves in a radial towards or away from a detector, the detected IR signals may be smaller than those generated by pets, which may result in a false positive output. The use of shields, such as umbrellas that block IR radiation can compound the problem, since the IR allocated to the “upper zone” may not receive a sufficient amount of radiation to detect the presence of an intruder in this case.
Regarding the processing of signals output by the detection devices, it is believed that U.S. Pat. No. 5,109,216 refers to adjusting the gain of an amplifier for processing the output of one detection channel. In addition, U.S. Pat. No. 5,578,988 refers to adjusting a microwave detection threshold based upon detection from another channel due to dynamic changes in the environment. However, it is also believed that the adjustment of amplitude described in these references may be insufficient to distinguish human intruders from pets.